In a sample of double-stranded DNA, 10 percent of the nitrogenous bases are guanine (G). What percentage of the nitrogenous bases in the sample are adenine (A) ?

40%

Explanation:

Remember that guanine always pairs with cytosine, and thymine always pairs with adenine. That means that any time there is a guanine, there is always a cytosine.

If there are 10% guanines, then there are 10% cytosines. That means the total of guanines and cytosines is 20%.

The other 2 bases (thymine and adenine) have to make up the rest of the bases. That means they have to make up 100-20 = 80%.

Again, there will be equal amounts of thymine and adenine, so 80%/2 = 40%.